A thermodynamic model was developed to describe phase equilibria for aqueous amino acid/polyelectrolyte gel systems. In the gel phase, activities of all exchangeable species (charged and neutral components) are calculated with the generalized Flory-Huggins model. In the surrounding solution, a numerical procedure is used to allow for partial dissociation phenomena combined with a modified UNIFAC model associated with rite Pitzer-Debye-Huckel term. The osmotic pressure difference between the two phases is also taken into account in the equations. The model enables the prediction of resin-phase composition, gel swelling and intraparticle pH. It can be used for various biochemicals in a wide range of concentrations. Its validity was successfully tested using binary and multicomponent exchange equilibria data of several amino acids (phenylalanine, alanine, proline, and glutamate) and HCl on a strong-acid cation-exchange resin (Amberjet 1200H).